A NOVEL METHOD FOR MEASURING AND CHARACTERIZING DYNAMIC FRACTURE-INITIATION TOUGHNESS OF ELASTIC-PLASTIC MATERIALS

The characterization and testing methods of the dynamic fractureinitiation toughness of elas- tic-plastic materials under tensileimpact are studied. By using the self-designed bar-bar tensile impactappa- ratus, a novel test method for studying dynamicfracture-initiation ahs been proposed based on the one-di- mensionaltest principle. The curve of average load v. s. displacement (P-δ)is smooth until unstable crack propagation, and the kinetic energywhich does not contribute to the crack growth can be removed fromtotal work done by external-force to the specimen.

Determination of dynamic modeⅠfracture toughness of rock at ambient high temperatures using notched semi-circular bend method

To investigate the influence of loading rate and high temperature on the dynamic fracture toughness of rock,dynamic fracture tests were carried out on notched semi-circular bend specimens under four temperature conditions based on the split Hopkinson pressure bar system.Experimental and analytical methods were applied to investigating the effect of temperature gradient on the stress waves.A high-speed camera was used to check the fracture characteristics of the specimens.The results demonstrate that the temperature gradient on the bars will not significantly distort the shape of the stress wave.The dynamic force balance is achieved even when the specimens are at a temperature of 400°C.The dynamic fracture toughness linearly develops with the increase of loading rate within the temperature range of 25-400°C,and high temperature has a strengthening effect on the dynamic fracture toughness.

Dynamic fracture toughness of high strength metals under impact loading:increase or decrease

An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1. For the purpose of verification, variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry, 30CrMnSiA and 40Cr, is studied in this work. Results of the experiments are compared, which were conducted on the modified split Hopkinson pressure bar （SHPB） apparatus, with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr, respectively. It is observed that for 30CrMnSiA, the crack tip loading rate increases with the increase of the striker velocity, while the fracture initiation time and the DFT simultaneously decrease. However, in the tests of 40Cr, there is also an increasing tendency of DFT, similar to other reports. Through an in-depth investigation on the relationship between the dynamic stress intensity factor （DSIF） and the loading rate, it is concluded that the generally increasing tendency in previous studies could be false, which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes. To disclose the real dependency of DFT on the loading rate, experimentsneed to be performed in a comparatively large striker velocity range.

DYNAMIC FRACTURE TOUGHNESS OF ULTRASTRENGTH STEELS FOR AIRCRAFT

The plots of load vs deflection(P-D)for two ultra-strength steels 30CrMnSiNi2A and 300M has been examined on a Charpy impact tester with digital memory and microcomputer. A criterion was found to availably represent the dynamic fracture toughness,K_(Ic)~D ,of the steels under impact loading.The K_(Ic)~D,.can be calculated with suggesting that the turning point of change in compliance on the P-D plot is the point of destabilized crack propagation.The K_(Ic)~D of steel 300M,quenched at 870℃ ,separately tempered at 300℃ and 450℃,was esti- mated to be 55—70 MN/m^(3/2)and 54—67 MN/m^(3/2)respectively.The K_(Ic)calculated with accuracy about 20% seems to be available to the engineering evaluation.

CRACK INITIATION POINT DETERMINATION AND DYNAMIC FRACTURE TOUGHNESS FOR CHARPY PRE-CRACKED SPECIMEN

The method to detect the crack initiation point of Charpy pre-cracked specimen under dy- namie loading was studied using strain gauge.The load-time curve and nominal strain-time curve at the crack tip for impact testing specinens may be shnultaneously measured by twin-channel oscilloscope with high speed sampling and diskette storing.Based on the dynam- ic finite element simulation of impact response of Charpy specimen,the measuring method of dynamic fracture toughness was analysed and some problems in previous dynamic fracture toughness measurement were discussed.

Improvement of Toughness of Ultrahigh Strength Steel Aermet 100

The influence of double aging on the microstructure and mechanical properties of ultrahigh strength steel Aermet 100 was analyzed. Under the double aging, there is no apparent decrease in the strength of steel. However, the impact fatigue life can be prolonged by 35.5% and dynamic fracture toughness be raised by 22.6% respectively, as compared with the normal aging. Based on the observation of microscopic structure, the physical mechanism of the prolongation of impact fatigue life and the enhancement of stability of the reverted austenite, AR, is analyzed further. The results show that this new technique is a breakthrough of combination optimization between strength and toughness for Aermet 100 steel. In the light of the current understanding on this subject, the volume fracture of soften and tough AR formed in process of heat preservation at higher temperature of double aging increases drastically. Moreover, during the treatment of lower temperature of double aging, the carbon separating from the martensitic ferrite will diffuse into AR, resulting that the martensitic brittleness decreases and the stability of AR increases.

Effect of loading rates on crack propagating speed,fracture toughness and energy release rate using single-cleavage trapezoidal open specimen under impact loads

The former studies indicate that loading rates significantly affect dynamic behavior of brittle materials,for instance,the dynamic compressive and tensile strength increase with loading rates.However,there still are many unknown or partially unknown aspects.For example,whether loading rates have effect on crack dynamic propagating behavior(propagation toughness,velocity and arrest,etc).To further explore the effect of loading rates on crack dynamic responses,a large-size single-cleavage trapezoidal open(SCTO)specimen was proposed,and impacting tests using the SCTO specimen under drop plate impact were conducted.Crack propagation gauges(CPGs)were employed in measuring impact loads,crack propagation time and velocities.In order to verify the testing result,the corresponding numerical model was established using explicit dynamic software AUTODYN,and the simulation result is basically consistent with the experimental results.The ABAQUS software was used to calculate the dynamic SIFs.The universal function was calculated by fractal method.The experimental-numerical method was employed in determining initiation toughness and propagation toughness.The results indicate that crack propagating velocities,dynamic fracture toughness and energy release rates increase with loading rates;crack delayed initiation time decreases with loading rates.

Application of split Hopkinson tension bar technique to the study of dynamic fracture properties of materials

A novel approach is proposed in determining dy- namic fracture toughness （DFT） of high strength steel, using the split Hopkinson tension bar （SHTB） apparatus, com- bined with a hybrid experimental-numerical method. The center-cracked tension specimen is connected between the bars with a specially designed fixture device. The fracture initiation time is measured by the strain gage method, and dynamic stress intensity factors （DSIF） are obtained with the aid of 3D finite element analysis （FEA）. In this approach, the dimensions of the specimen are not restricted by the connec- tion strength or the stress-state equilibrium conditions, and hence plane strain state can be attained conveniently at the crack tip. Through comparison between the obtained results and those in open publication, it is concluded that the ex- perimental data are valid, and the method proposed here is reliable. The validity of the obtained DFT is checked with the ASTM criteria, and fracture surfaces are examined at the end of paper.

Advances in dynamic test of deep rocks considering in-situ mechanical and hydraulic conditions

Determining the dynamic behaviors of deep rocks is important but challenging for deep underground rock engineering.Recently,great advances have been made by researchers in dynamic tests of deep rocks considering insitu mechanical and hydraulic conditions.It is thus imperative to systematically review the progress in this field.This paper focuses on the dynamic experiments of deep rocks using the modified split Hopkinson pressure bar(SHPB),including new advances in experimental apparatuses,methodologies,and results.A comprehensive introduction to the principles of the triaxial confinement SHPB,the true triaxial SHPB,and SHPB with a coupled hydraulic-mechanical device is provided.The development of these systems for conducting dynamic tests of deep rocks is briefly presented.Moreover,the experimental methods and results for quantifying dynamic compressive,tensile,flexure,and shear strengths and mode I/II fracture toughness of rocks under axial preload,hydrostatic pressure,triaxial stress state,and coupled hydraulic-mechanical condition are systematically discussed,and the rate-dependent characteristics and mechanisms of deep rocks under various geological occurrence conditions are also summarized.

Numerical study of the semi-circular bend dynamic fracture toughness test using discrete element models

The semi-circular bend(SCB) dynamic fracture toughness test is simulated using discrete element models. The influence of the frictional boundary condition, constitutive law and specimen thickness on the test measurements is investigated. It is found that friction between loading plates and the rock specimen affects the test results. Therefore, friction must be carefully considered to obtain accurate measurements. The simulation results also show that in contrast to the 2D model in which a rate-dependent cohesive law must be introduced, 3D models with a rate-independent law can produce good results. Furthermore, the study suggests that test measurements are seriously affected by specimen thickness; thus, full 3D modeling is required for simulation of the SCB test.

Dynamic Fracture Toughness of Armour Grade Quenched and Tempered Steel Joints Fabricated Using Low Hydrogen Ferritic Fillers

The armour grade quenched and tempered steel joints fabricated using low hydrogen ferritic steel （LHF） filler exhibited superior joint efficiency owing to preferential ferrite microstructure in the welds and also they offered required resistance to HIC. However, the combat vehicles used in military operations will be required to operate under a wide range of road conditions ranging from first class to cross country. Structural components in combat vehicles are subjected to dynamic loading with high strain rates during operation. Stress loadings within the vehicle hull of these vehicles are expected to fluctuate considerably and structural cracking especially in welds during the service life of these vehicles can lead to catastrophic failures. Under these conditions, fracture behaviour of high strain rate sensitive structural steels can be better understood by dynamic fracture toughness （K1d）. Hence, an attempt was made to study dynamic fracture toughness of the armour grade quenched and tempered steel and their welds fabricated using LHF consumables. The experimental results indicate that the K1d values of the joints fabricated by shielded metal arc welding （SMAW） are higher than those of the joints fabricated using flux cored arc welding （FCAW） process.